New York City has some strange smells, especially in the subway. Walking underground you can sense that the air just feels stuffier, smells smellier, and must be dirtier.

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Well according to new research, the microbial population in the air of the New York City subway system is nearly identical to that of ambient air on the city streets.

The air we breathe is loaded with bacteria, which has potential to be harmful to human health. Regulatory focus on air quality has been on chemical and particulate materials, and microorganisms and microbial products which are prevalent in public settings are often overlooked.

This research establishes an important baseline, should it become necessary to monitor the subway's air for dispersal of potentially dangerous microbes, including those associated with bioterrorist attacks.

The paper states that although subways might be considered confined environments, the similarity of subway air microbiota to that of outside air suggests that the subway air significantly equilibrates with outside air on relatively short timescales. "There is little or no local air conditioning in the NYC subway. Instead, air movement in the system is driven by passive train-pumping, with air taken in and exhausted through street-level ports, the NYC sidewalk grillwork. The general uniformity of microbial assemblages throughout the system indicates good air mixing."

The results "are strong testimony for the efficiency of the train pumping system for ventilation," says principal investigator Norman R. Pace of the University of Colorado, Boulder. The wind one feels while walking across a subway grate as the subway clatters beneath also demonstrates just how effective that system is, he says. The only obvious differences in the subway's microbial population are the somewhat higher proportion of skin microbiota, and the doubled density of the fungal population, which Pace suggests may be due to rotting wood.

The researchers collecting a sufficient volume of air—a couple of cubic meters—to take the bacterial census within 20 minutes, instead of after "hours," says Pace.

Pace notes that until now, the microbial content of subway air was unknown, and that the microbiology of indoor air is an emerging field of scientific inquiry.

"While it is difficult to predict what will be discovered on the frontier of scientific inquiry, the opportunity exists to better understand these complex microbial ecosystems and how they affect health and the environment. We expect that someday this knowledge will influence design and construction practices and other industrial processes," says Paula Olsiewski, program director, Alfred P. Sloan Foundation.